Self-thread-forming fastener with easy thread pickup

ABSTRACT

The work-entering end of a self-thread-forming fastener is provided with a step-tapered portion for initially engaging the sidewall of the pilot hole of a workpiece. The invention is characterized by the fact that the tapered portion is of substantially circular cross section while the radial height of the thread over such step-tapered portion increases from a beginning height less than the diameter of the pilot hole to the full height of the thread on the shank portion in stages. In the first such stage, the crest of the thread increases rapidly in radial height within a small circumferential angle to establish substantial initial thread-forming engagement with the pilot hole. In the second stage, the crest of the thread continues for at least one full thread turn at substantially the same radial height as it achieves in the first stage to insure thread pickup with minimum end loading. In the third stage, the crest of the thread increases in radial height substantially to the maximum radial height of the crest of the thread on the shank portion.

United States Patent [72] Inventor EugeneR. Larson Rockford, Ill. [21]Appl.No. 11,211 [22] Filed Feb. 13, 1970 [45] Patented Jan. 11, 1972[73] Assignee Textron Inc.

Rockford, Ill.

[54] SELF-THREAD-FORMING F ASTENER WITH EASY Primary Examiner- Ramon S.Britts Altorney-Buckhorn, Blore, Klarquist and Sparkman ABSTRACT: Thework-entering end ofa self-thread-forming fastener is provided with astep-tapered portion for initially engaging the sidewall of the pilothole ofa workpiece. The invention is characterized by the fact that thetapered portion is of substantially circular cross section while theradial height of the thread over such step-tapered portion increasesfrom a beginning height less than the diameter of the pilot hole to thefull height ofthe thread on the shank portion in stages. 1n the firstsuch stage, the crest of the thread increases rapidly in radial heightwithin a small circumferential angle to establish substantial initialthread-forming engagement with the pilot hole. In the second stage, thecrest of the thread continues for at least one full thread turn atsubstantially the same radial height as it achieves in the first stageto insure thread pickup with minimum end loading. 1n the third stage,the crest of the thread increases in radial height substantially to themaximum radial height of the crest of the thread on the shank portion.

PATENTEDJMH 1 I972 3533455 SHEET 1 or 2 [will] II EUGENE R. LARSONm/vmrm B) BUG/(HORN, BLORE, KLAROU/ST a SPAR/(MAN ATTORNEYS PATENTEDJAN! 1 m2 3.8333155 SHEET 2 [1F 2 SELF-THREAD-FORMING FASTENER WITIIEASY THREAD PICKUP BACKGROUND OF THE INVENTION A problem withself-thread-forming fasteners previously known has been that suchfasteners have frequently required a heavy end loading to prevent theforward ends of the thread swaging portions thereof from reaming out theforepart of a pilot hole at the commencement of the tapping of suchhole. A heavy required end load is obviously a disadvantage in drivingself-tapping screws due to the tiring physical effort involved inassembly operations.

It is thus the principal object of the present invention to provide aself-thread-forming fastener with a work-entering end portion that willminimize the amount of end loading required to engage the thread swagingsection into the sidewall of a pilot hole in a workpiece.

It is a further object of the present invention to provide such afastener with a pilot hole sidewall-engaging thread portion having atleast one full thread turn of substantially circular cross-sectionalconfiguration at its work-entering end thereby to maintain such minimalrequired end loading for at least 360 or until the heavier threads onthe shank portion make contact.

It is a still further object of the present invention to provide such aself-thread-forming fastener wherein the substantially circularcross-sectional configuration of the thread on the work-entering endmaintains the required driving torque at a low level while providingsufficient bite to start the cutting of the thread.

It is a still further object of the present invention to provide a blankfor forming such a self-thread-forming fastener.

SUMMARY OF THE INVENTION In accordance with the present invention, Iprovide a selfthread-forming fastener for forming threads in a pilothole in a workpiece, the fastener having a step-tapered work-enteringend portion, a shank portion, and a thread continuing from the shankportion onto the work-entering end portion.

The fastener is characterized by the fact that the thread turn on thetapered portion which is adapted for initially engaging the sidewall ofthe pilot hole in the workpiece has a crest of substantially circularcross section and a radial dimension greater than the radius of thepilot hole by an amount equal to approximately to 30 percent of theheight of the thread on the shank portion. Such thread turn continues atthe aforesaid radial dimension for at least one full thread turn, andpreferably for approximately 1% thread turns, and thereafter graduallychanges in dimension to the dimensions of the thread on the shankportion.

In accordance with the invention, the work-entering end portion of theblank from which the fastener is made is steptapered. The crest of thethread over such step-tapered portion thus increases in radial heightfrom a beginning height less than the diameter of the pilot hole to itsfull height at the shank portion of the fastener in stages. In the firststage the crest of the thread increases rapidly in radial height fromthe aforementioned beginning height within a small circumferentialangle, less than one-half a thread revolution, and preferably withinapproximately 60, to a radial height approximately 20 to 30 percent offull thread height greater than the radial dimension of the pilot hole,to establish substantial initial thread-forming engagement with thesidewall of the pilot hole in the workpiece.

In the second stage, the crest of the thread continues for at least onefull thread turn at substantially the same maximum radial height as itachieves in the first stage. In the third stage, the crest of the threadincreases in radial height substantially to that of the crest of thethread on the shank portion.

The method of the invention comprises forming a blank having a shankportion, a work-entering end portion of circular cross section having adiameter smaller than the diameter of the pilot hole, and anintermediate cylindrical portion LII between the shank portion and thework-entering end portion. The intermediate cylindrical portion is alsoof circular cross section and has a length equal to at least one fullthread turn and preferably 1% thread turns, and a diameter intermediatethat of the shank portion and the work-entering end portion.

The blank further comprises other portions which taper conicallyinwardly from the intermediate cylindrical portion to the work-enteringend portion and outwardly from the intermediate portion to the shankportion, thereby to form conical transitional portions which achieve theunique thread configuration above described. The blank is then rolled ona centerless basis to form the continuous thread.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view ofablank used in the manufacture of a fastener device according to thepresent invention and having a shank portion of arcuate triangular crosssection.

FIG. 2 is a sectional view taken on line 2-2 of FIG. I.

FIG. 3 is a side elevational view of a self-thread-forming fastenerconstructed in accordance with the present invention and made from theblank illustrated in FIG. 1.

FIG. 4 is an end view to an enlarged scale of the fastener deviceillustrated in FIG. 3 and showing in full lines the outline of the majordiameter of the pilot end of the fastener and the major diameter of thethreaded shank portion, the threads on the step-tapered intermediateportion being omitted in the interest of clarity. The dotted lineindicates the circumscribing circle on the shank portion, which is equalto the root diameter of the tapped hole.

FIG. 5 is a sectional view taken on line 55 of FIG. 4, the dotted linesshowing the configuration of the original blank from which the fasteneris constructed.

FIG. 6 is a sectional view taken on line 66 of FIG. 3, the dotted linesalso showing the configuration of the original blank.

FIG. 7 is a schematic sectional view taken substantially on line 77 ofFIGS. 5 and 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In this application thefollowing definitions shall be applicable.

Pitch diameter" is used as a generic term to designate the diameter,i.e., maximum transverse width of any section of either the pitchcylinder" or the pitch cone" as determined by the three-wire method ofpitch diameter measurement. Due to the triangular nature of the shankportion of the particular fastener herein disclosed, the pitch diameter,i.e., maximum transverse width, does not always extend through the axialcenter of the configuration.

The three-wire method of pitch diameter measurement is well known in theart and is described, for example, in US. National Bureau of StandardsHandbook No. H28 (I957).

Pitch cylinder is, on a straight thread, an imaginary coaxial cylinderthe surface of which would pass through the thread profiles, or theprojection thereof, at such points as to make the width of the groove,or the projection thereof, equal to one-half the basic pitch. Due to thetriangular configuration of the cross section of the shank portion ofthe particular fastener herein disclosed, the pitch cylinder thereof isnot round but of arcuate triangular cross-sectional shape.

Pitch cone on a taper thread is an imaginary coaxial cone, the surfaceof which would pass through the thread profiles, or the projectionthereof, at such points as to make the width of the groove or theprojection thereof equal to one-half the basic pitch.

Pitch surface is used herein to designate the surface of revolution ofthe pitch line defining either the pitch cylinder or the pitch cone ashereinbefore defined.

Pitch surface cross section" is used herein to designate the transversecross section of any pitch surface, such as that of either the pitchcylinder or the pitch cone, as hereinbefore defined. For reasonsmentioned above, it may be of arcuate triangular configuration.

Radial height is the distance measured from the axis of the fastener tothe crest of the thread.

Thread height" is the distance measured perpendicularly to the axisbetween the root and the crest of the thread.

Referring now to the drawings, a self-thread-forming fastener accordingto the present invention is illustrated in the form of a dog-point,self-thread-forming fastener having a head 11 at one end thereof, ashank portion 12, a threaded dog-point l4, and a step-taperedthread-forming intermediate portion 16. While in the illustratedembodiment the shank portion 12 is of arcuate triangular configuration,it is to be understood that the invention is not to be limited to anyparticular configuration of the shank portion, which may be of anywell-known cross-sectional shape. A continuous thread is provided on thesections l2, l4 and 16. The dog-point I4 is provided to facilitatedriving the fastener 10 straight; that is, in coaxial alignment with ahole 18 in a workpiece 20. The steptapered intermediate portion 16 isthe thread swaging section which forms the thread in the sidewalls ofthe hole 18 in the workpiece 20.

FIGS. 1 and 2 illustrate a blank 22 from which the fastener 10illustrated in FIG. 3 is made. The blank 22 comprises a shank portion 24of arcuate triangular cross section, and a work-entering end portion 26of circular cross section having a diameter smaller than the diameter ofthe pilot hole 18. The portion 26 as illustrated is approximately fourthread turns in length, although it may be made as long as the specificapplication requires or it may be substantially omitted entirely.

A step-tapered intermediate portion 28 is provided between the shankportion 24 and the work-entering end portion 26. Such intermediateportion 28 itself comprises an intermediate circular cylindrical portion30 having a length equal to at least one full thread turn and preferably1%; thread turns. The diameter of the cylindrical portion 30 is slightlygreater than the diameter of the portion 26 and, as shown in FIG. 2, issubstantially equal to that of the circle 32 that can be inscribedwithin the arcuate triangular shank portion 24. In a fastener having acircular shank, the diameter of the intermediate portion is intermediatethat of the shank portion and the work-entering end portion. Theintermediate portion 28 further comprises a portion 34 which tapersconically inwardly from the portion 30 to the work-entering end portion26 over a length approximating two-thirds of a thread turn. Anotherportion 37 tapers coni cally outwardly from the portion 30 to the shankportion 24 over a length approximating two thread turns when measured toits intersection with the arcuate triangular shank portion 24 at a lobe36. The overall length of the blank as well as the relative dimensionsof the portion 28 are determined by the requirements of each specificapplication. Rolling the blank 22 on a centerless basis between dies(not shown) forms the continuous thread.

Referring particularly to FIGS. 5 and 6, the dog-point 14 has severelyunderfilled threads 38 whose crest diameter is less than the diameter ofthe hole 18. Such underfilled crests 38 increase in height over thestep-tapered portion l6 to the completely filled threads 40 of thearcuate triangular shank portion 12.

A feature of the invention is the fact that the increase in radialheight of the thread from its beginning height as at 41 on the dog-point14 to its full height at 40 on the shank portion 12 occurs in stages. Inthe first stage, and as shown in FIGS. 5, 6 and 7, the crest 43 of thethread increases rapidly in radial height within a small circumferentialangle, such as, for example, 60, from point 44 to establish substantialinitial threadforming engagement at point 46 with the sidewall of thehole 18. Such increase can be noted by comparing the height of thethread at 44 in FIG. 5 which fits within the hole 18 with the height ofthe same thread at 46 in FIG. 6 only 60 away.

The difference in radial height between the crests of the threads at 44and 46, which difference is indicated by the numeral 47 in FIG. 6,arises from the increased penetration of the roll thread dies into thetapered portion 34 of the blank 22. Provided threads are underfilled,the greater the penetration of the dies into the surface of the blank,the higher will be the resultant major dimension as measured over thedisplaced stock. FIG. 6 is a section taken through the fastener 10 inline with a low area 48 intermediate the lobe crests 36 on the shankportion 24. The penetration of the dies into this portion of the blankresults in a greater thread crest diameter at the intersection of theconical portion 34 and the cylindrical portion 30 of the blank 22 thanthe crest diameter of the thread 4! on the blank portion 26. a

The radial dimension of the thread at 46 is greater than the radius ofthe hole 18 by an amount equal to approximately 20 to 30 percent of theheight of the thread 40 on the shank portion 12. Such difference inheight is also indicated by the numeral 47 in FIG. 6. The crest of thethread at 46 thereafter continues in a second stage substantially atthis greater radial height along the intermediate circular cylindricalportion 30 of the blank 22 for at least one full thread turn and in theembodiment illustrated, for 1% thread turns. Thereafter and in a thirdstage, the thread gradually changes in height and also in cross sectionalong the conical portion 37 from its substantially circular crosssection as illustrated by the dotted line 46a in FIG. 7 to an arcuatetriangular configuration at 42 as it merges with the thread 40 on theshank portion 12.

In operation the threads 38 on the dog-point 14 provide the axialstability required for the subsequent straight driving of the fastener.Free entry into the hole is prevented by the uptaper of the crest 43 ofthe thread as it increases in radial height from its height at 44 to itsheight at 46. Whereas the radial height of the crest of the thread at 46is substantially greater than the radial dimension of the pilot hole 18,the crest of the same thread only 60 away at 44 is actually in the hole18.

At this stage in the driving of the fastener 10, the dog-point 14 comesinto play. The stability in the hole which the point 14 provides permitsend loading of the head 11 of the fastener without danger of tipping itover or crooked starting into the hole.

As the fastener is turned with a slight amount of end loading, theincreasing height of the crest 43 of the thread between 44 and 46 beginsto tap a matching thread groove of relatively shallow depth in thesidewall of the workpiece. Such shallow depth is maintained for at leastone full thread turn. Due to the low height of the thread 46a across theintermediate circular cylindrical portion 30, little effort is requiredto engage it into the sidewall of the hole. The provision of at leastone full thread turn of this substantially circular thread maintains therequired torque at a low level but provides enough bite" to start thetapping of the hole with low-pressure end loading.

As .the third stage of the thread on the conical portion 37 is engaged,such thread changes gradually from its substantially circular crosssection at 46 to an arcuate triangular configuration at 42 whence itmerges with the thread 40 on the shank portion 12. The arcuatetriangular shape of such gradually changing thread completes the threadswaging operation that was started by the threads along the first andsecond stages.

Due to the arcuate triangular cross-sectional shape of thethread-forming thread turns on the tapered section 37 of the blank, thedriving torque requirement is held relatively low, notwithstanding thatthese thread turns penetrate the workpiece to maximum depth. It is to beunderstood that the thread turn of the second stage, which engages thesidewall of the workpiece throughout substantially 360 tends to draw orpull the fastener into the workpiece thus reducing the amount of manualend loading required for driving the third stage into the workpiece.Moreover, reaming of the mouth of the pilot hole in the workpiece issubstantially eliminated. Not until the thread 40 on the shank portion12 is fully engaged does the full sized triangular portion 12 begin toenter the hole. By this stage, however, the tapping of the hole to itsroot diameter 50 has been completed, and subsequent rotation of thescrew with minimum end loading will drive it to the depth required.

While I have described my invention with respect to the manufacture ofone particular embodiment, it should be realized that the inventionpermits numerous modification as to arrangement and detail. l thereforeclaim as my invention all such modifications as come within the truespirit and scope of the following appended claims.

What is claimed is:

1. A self-thread-forming fastener for forming threads in a pilot hole ina workpiece and having a pilot end of circular cross section of smallerdiameter than said pilot hole, a threaded shank portion of larger pitchsurface cross section than said pilot end and a step-taperedthread-forming portion of circular cross section intermediate said pilotend and said shank portion,

a. the crest of the thread over said step-tapered portion increasing inradial height from a beginning height at said pilot end less than theradius of said pilot hole to its full height in said shank portion inthree stages;

b. in the first stage the crest of the thread increasing rapidly inradial height within a small circumferential angle, less than one-half athread revolution, to establish substantial initial thread-formingengagement with the sidewall of said pilot hole in said workpiece;

c. in the second stage the crest of the thread continuing for at leastone full thread turn at substantially the same radial height as itachieves in said first stage; and

d. in the third stage the crest of the thread increasing in radialheight substantially to the maximum radial height of the crest of thethread on said shank portion.

2. A self-thread-forming fastener as in claim 1 in which said shankportion has an arcuate triangular pitch surface cross section and saidrapidly increasing radial height of said crest of said thread in saidfirst stage occurs at a point on said steptapered portion in line withan intermediate low area on said shank portion.

3. A self-thread-forming fastener as in claim 2 in which said crest ofsaid thread in said first stage increases rapidly in radial height inapproximately 60.

4. A self-thread-forming fastener as in claim I in which said crest ofsaid thread continues in said second stage at a substantially constantradial height for approximately l /3 thread turns.

5. A self-thread-forming fastener for forming threads in a pilot hole ina workpiece, said fastener having a tapered worlcentering end portionand a shank portion, the shank portion being provided with a thread ofarcuate triangular pitch surface cross section, the thread continuingfrom said shank portion onto said work-entering end portion, theinvention being characterized by:

a. the thread turn on said tapered portion which is adapted forinitially engaging the sidewall of the pilot hole in the workpiecehaving a crest of substantially circular cross section and a radialdimension greater than the radius of said pilot hole by an amount equalto approximately 20 to 30 percent of the height of the thread on saidshank portion;

b. said first-mentioned thread continuing at the aforesaid radialdimension for at least one full thread turn; and

c. said thread on said tapered portion following said one full threadturn changing gradually from said substantially circular cross sectionconfiguration to an arcuate triangular configuration whence it mergeswith said thread on said shank portion.

1. A self-thread-forming fastener for forming threads in a pilot hole in a workpiece and having a pilot end of circular cross section of smaller diameter than said pilot hole, a threaded shank portion of larger pitch surface cross section than said pilot end and a step-tapered thread-forming portion of circular cross section intermediate said pilot end and said shank portion, a. the crest of the thread over said step-tapered portion increasing in radial height from a beginning height at said pilot end less than the radius of said pilot hole to its full height in said shank portion in three stages; b. in the first stage the crest of the thread increasing rapidly in radial height within a small circumferential angle, less than one-half a thread revolution, to establish substantial initial thread-forming engagement with the sidewall of said pilot hole in said workpiece; c. in the second stage the crest of the thread continuing for at least one full thread turn at substantially the same radial height as it achieves in said first stage; and d. in the third stage the crest of the thread increasing in radial height substantially to the maximum radial height of the crest of the thread on said shank portion.
 2. A self-thread-forming fastener as in claim 1 in which said shank portion has an arcuate triangular pitch surface cross section and said rapidly increasing radial height of said crest of said thread in said first stage occurs at a point on said step-tapered portion in line with an intermediate low area on said shank portion.
 3. A self-thread-forming fastener as in claim 2 in which said crest of said thread in said first stage increases rapidly in radial height in approximately 60*.
 4. A self-thread-forming fastener as in claim 1 in which said crest of said thread continues in said second stage at a substantially constant radial height for approximately 1 1/3 thread turns.
 5. A self-thread-forming fastener for forming threads in a pilot hole in a workpiece, said fastener having a tapered work-entering end portion and a shank portion, the shank portion being provided with a thread of arcuate triangular pitch surface cross section, the thread continuing from said shank portion onto said work-entering end portion, the invention being characterized by: a. the thread turn on said tapered portion which is adapted for initially engaging the sidewall of the pilot hole in the workpiece having a crest of substantially circular cross section and a radial dimension greater than the radius of said pilot hole by an amount equal to approximately 20 to 30 percent of the height of the thread on said shank portion; b. said first-mentioned thread continuing at the aforesaid radial dimension for at least one full thread turn; and c. said thread on said tapered portion following said one full thread turn changing gradually from said substantially circular cross section configuration to an arcuate triangular configuration whence it merges with said thread on said shank portion. 